Use a strong light and a magnifying glass to see whether the voice inlet hole in front of the microphone may be blocked.
If it is (or even if you can’t tell), try using a vacuum cleaner to clear the hole of any blockage.
DO NOT use a pin or probe to try and clear the hole as you may damage the microphone.
Here’s an image which shows the location of the voice inlet hole (and the microphone position in the tablet).
(click on image to enlarge for better viewing).
If doing this doesn’t resolve the problem you may have to open the tablet so that you can access the case from both sides to visually inspect that the voice inlet hole is unobstructed.
If it is blocked then it will be easier to clear without causing any damage to the microphone.
Here’s a link to the ifixit LeapFrog LeapPad Platinum Motherboard Replacement guide. It is useful as it shows how to open the tablet so that access to the voice inlet is possible.
If the voice inlet hole is clear then there may be a problem with the microphone itself.